"The tide is perceptible five hundred miles from the sea; it does not carry the salt water up with it, but there is simply a rise and fall of the fresh water. So great is the volume of the Amazon where it enters the sea that ships can dip up fresh water while yet out of sight of land." "In speaking of the tide," said the Doctor, "don't forget to mention the _piroróco_ or 'bore' of the Amazon." "I was just coming to it," replied the youth, "and cannot do better than quote a description by La Condamine, written more than a hundred years ago. Here it is: "'During three days before the new and full moons, the period of the highest tides, the sea, instead of occupying six hours to reach its flood, swells to its highest limits in one or two minutes. The noise of this terrible flood is heard five or six miles, and increases as it approaches. Presently you see a liquid promontory, twelve or fifteen feet high, followed by another and another, and sometimes by a fourth. These watery mountains spread across the whole channel, and advance with a prodigious rapidity, rending and crushing everything in their way. Immense trees are instantly uprooted by it, and sometimes whole tracts of land are swept away.'" "It must be a terrible thing for boats to encounter, especially the small ones," Fred remarked, as Frank concluded the above description. "It is," Dr. Bronson answered, "and many of them are lost every year. But those engaged in navigating the river know when to expect the bore, and take precautions against it. They have _esperas_, or resting-places, where they are sheltered from its force, and wait until it has passed. "The bore is not confined to the Amazon," continued the Doctor; "it is known in other rivers, especially in the Hoogly, below Calcutta, but the bore of the Amazon is undoubtedly the largest." [Illustration: IN AN IGARIPÉ.] "Another curious feature of the Amazon," said Frank, resuming, "is the great number of lateral channels, which are technically called _igaripés_, or canoe-paths. Boats may go for hundreds of miles along the lower Amazon in the _igaripés_ without once entering the main stream. They remind us of the bayous of the lower part of the Mississippi Valley." "Don't forget," said Fred, "that the Amazon rises within sixty miles of the Pacific Ocean, and touches every country of South America except Chili and Patagonia. The Madeira rises close to the sources of the La Plata, while the Negro, the great northern tributary of the Amazon, is connected with the Orinoco by a navigable canal called the Cassiquari. The navigation of this network of waters is favored by nature; the current is eastward, while the trade wind blows west from the Atlantic, so that ships going either way have the stream or the wind to help them along."